A New Family of P,N Chelates: Stereoselective Synthesis of 2-Pyridyl-2-phospholenes in the Coordination Sphere of Palladium(II) Complexes

The synthesis of Pd(II) complexes 2a,c‘, bearing 2-(2-pyridyl)phosphole ligands which act
as P,N chelates, is described. Ligands featuring a pendant pyridyl group spontaneously evolve
to the corresponding 2-pyridyl-2-phospholene derivatives in the coordination sphere of Pd(II).
This isomerization is a stereoselective process, the stereochemistry of the resulting derivatives
3a,a‘ being established by X-ray diffraction studies. The isomerization of ligands lacking
pendant pyridyl groups was accomplished by adding pyridine to the reaction media. High
yields were achieved under very mild conditions for ligands possessing different substitution
patterns. Multinuclear NMR spectroscopic data and X-ray diffraction studies showed that
the stereoselectivity of the process is preserved under these new reaction conditions. The
transformation did not occur with free 2-(2-pyridyl)phospholes, even at high temperatures
in the presence of pyridine. DFT calculations confirmed that coordination to a Pd(II) center
is required to make this isomerization thermodynamically feasible. The isomerization has
also been demonstrated with Pt(II) precursors but failed with CuCl, CuCl2, and ZnCl2. The
free 2-pyridyl-2-phospholenes 4a,c‘ were obtained by reacting the complexes 3a,c‘ with dppe.
No inversion of the P atom of the phospholene ring was observed up to 90 °C, indicating
that this novel family of P,N chelates is a promising class of ligand for homogeneous catalysis.